In a standard infrastructure of a wireless Local Area Network, which is a centralised network, all client stations are in the range of, and attach to an Access Point acting as a master station. In mesh networks, however, not all the stations are within the hearing range of each other, namely radio meshes comprise many hidden terminals. Accordingly, in such networks, there is a risk of seeing interference with ongoing transmissions. Thus, data transmission methods used in standard infrastructure network turn out to be inefficient in mesh networks.
To remedy this issue, it has been proposed in IEEE 802.11 to use Network Allocation Vectors (NAVs) to inhibit medium access of a station when another transmission is already going on. A NAV is set and reset by data transmissions or management frame transmissions. However, it has been noted that this NAV mechanism is not well suited when multiple, simultaneous reservations are pending. Let's detail this deficiency based on an exemplary network comprising five stations A, B, C and D. Station C uses the NAV to inhibit medium access. Stations A and B have little data to transmit and station E is the owner in a reservation by D and E. As stations A and B have little data to transmit, they may foreshorten their transmission. However, existing mechanisms for foreshortening transmission reset the NAV, so that stations C is no longer blocked and can access the medium. Doing so, however, it interferes with the ongoing transmission from D to E, which will be corrupted by any transmission attempt by C.
To avoid such interference, it could obviously be possible to make the NAV permanent, so that it cannot be reset. With this solution, any compliant station sets its NAV as soon as any reservation period starts and goes to a sleep mode until the end of the reservation period, or until the end of the last reservation period if there are multiple, consecutive, reservations. However, this has a second drawback that lots of airtime can be wasted in case a station having reserved the medium, does not have any data to transmit.